The assignee of the present invention manufactures and deploys spacecraft for, inter alia, communications and broadcast services.
Such spacecraft are equipped with on board propulsion systems, including chemical or electric thrusters, for orbit raising from a launch vehicle transfer orbit (or “parking orbit”) to an operational orbit, for example, to a geosynchronous orbit; for stationkeeping once disposed in the operational orbit; and for attitude control/momentum management purposes.
Some known art related to such satellite propulsion systems include U.S. Pat. Nos. 8,763,957, 7,113,851, 6,543,723, 9,108,748, and 9,108,749, all assigned to the assignee of the present invention.
The propulsion mission functions contemplated by the present application, include, but are not limited to, momentum management and orbit control, orbit control including orbit raising, orbit lowering and stationkeeping (N-S and E-W) for geosynchronous and other Earth orbiting spacecraft. Typical requirements for such propulsion mission functions are described in detail in U.S. Pat. No. 6,032,904, assigned to the assignee of the present invention, and may be summarized as follows.
Orbit raising functions relate to the task of transferring a spacecraft from an initial lower orbit (into which the spacecraft has been injected by a launch vehicle) to, for example, an intermediate orbit or an operational orbit or from an operational orbit to a graveyard orbit. Where a liquid chemical thruster is the propulsion technology chosen for performing the orbit raising function, the mass of the chemical propellant needed for orbit raising can be as much as half of the spacecraft total mass injected into the initial orbit by the launch vehicle. Where an electric thruster system is used for part or all of the orbit raising function, a substantial mass savings may be achieved, by virtue of the electric thruster's higher specific impulse (Isp) however, significantly more time must be allocated to the orbit raising phase of the spacecraft's life, as a result of the electric thruster's lower thrust. Orbit lowering functions relate to the task of transferring a spacecraft from an initial higher orbit to a lower orbit.
Once in an operational orbit, the propulsion system is responsible for maintaining correct orbital position and attitude throughout the life of the spacecraft. For a geostationary spacecraft, for example, the correct orbital position always lies in the plane of the earth's equator, at a particular assigned longitude. Various forces act on the spacecraft which, in the absence of propulsion stationkeeping functions, tend to move the spacecraft out of the desired orbital position. These forces arise from several sources including the gravitational effects of the sun and moon, the elliptical shape of the earth, and solar radiation pressure. Stationkeeping includes control of the inclination, eccentricity, and drift of the spacecraft. The orbit's inclination relates to the north-south position of the spacecraft relative to the earth's equator and may be maintained at a value acceptably close to zero by performing periodic north-south stationkeeping (NSSK) maneuvers. Drift is a measure of the difference in longitude of the spacecraft's sub satellite point and the desired geostationary longitude as time progresses and may be corrected by performing periodic east-west stationkeeping (EWSK) maneuvers. Eccentricity is a measure of the noncircularity of the spacecraft orbit, and may be controlled in the course of performing NSSK and/or EWSK maneuvers, or separately.
Once on-station, a spacecraft must maintain its attitude in addition to its orbital position. Disturbance torques, such as solar pressure, work to produce undesired spacecraft attitude drift. Momentum wheel stabilization systems are commonly used to counteract such disturbance torques. Such systems typically include one or more momentum wheels and control loops to sense and control changes in the spacecraft attitude. The control loops determine the required speed of the wheels to absorb or off-load momentum based on a sensed spacecraft attitude. Momentum stored in the momentum wheels must be periodically unloaded, to keep the momentum wheels within a finite operable speed range. Momentum wheel unloading is typically accomplished by applying an external torque to the spacecraft by firing a thruster, a propulsion mission function referred to herein as momentum management.
Electric propulsion is being used with increased frequency for both orbit raising and orbital stationkeeping. For many final orbits, including particularly geosynchronous orbits, orbit raising and NSSK maneuvers are the dominant propulsion activity. Efficiency of a NSSK maneuver by aligning the thrust vector to provide that at least a substantial component of the thrust vector is in the orbit normal direction, i.e., parallel to the axis of rotation of the solar arrays, which must counter orbital rotation to maintain the arrays in a sun-oriented attitude.
During orbit raising, however, the satellite must be steered along a non-fixed attitude trajectory to optimize the use of propellant and minimize orbit raising time of flight. In order to keep the solar arrays in a sun-oriented attitude during orbit raising, a combination of rotation of the satellite about the thrust vector and rotation of the arrays around their rotational axis simultaneously maintains sun tracking and thrust vector tracking. Thus, orbit raising generally requires the thrust vector to be at least approximately orthogonal to the axis of rotation of the solar arrays.
In the absence of the presently disclosed techniques, the competing requirements to produce thrust orthogonal to the solar array rotational axis during orbit raising and parallel to the rotational axis during NSSK have been met by providing additional thrusters and/or complex actuators to reposition a thruster.